Active photosensitive structure with buried depletion layer

ABSTRACT

An imager pixel has a photosensitive JFET structure having a channel region located above a buried charge accumulation region. The channel region has a resistance characteristic that changes depending on the level of accumulated charge in the accumulation region. During an integration period, incident light causes electrons to be accumulated inside the buried accumulation region. The resistance characteristic of the channel region changes in response to a field created by the charges accumulated in the accumulation region. Thus, when a voltage is applied to one side of the channel, the current read out from the other side is characteristic of the amount of stored charges.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor devices and,in particular, to an active pixel photosensitive structure.

BACKGROUND OF THE INVENTION

A CMOS imager includes a focal plane array of pixel cells, each cellincludes a photosensor, for example, a photogate, photoconductor or aphotodiode overlying a substrate for producing a photo-generated chargein a doped region of the substrate. Typical CMOS imager pixel cells haveeither a three transistor (3T) or four transistor (4T) design. The 4Tdesign is preferred over the 3T because it reduces the number of “hot”pixels in an array (those that experience increased dark current), andit diminishes the kTC noise that 3T designs may experience with thereadout signals.

In a CMOS imager, the active elements of a pixel cell, for example afour transistor pixel, perform the necessary functions of (1) photon tocharge conversion; (2) transfer of charge to a floating diffusionregion; (3) resetting the floating diffusion region to a known statebefore the transfer of charge to it; (4) selection of a pixel cell forreadout; and (5) output and amplification of signals representing areset voltage and a pixel signal voltage, the latter based on the photoconverted charges. The charge at the floating diffusion region isconverted to a pixel output voltage by a source follower outputtransistor.

Exemplary CMOS imaging circuits, processing steps thereof, and detaileddescriptions of the functions of various CMOS elements of an imagingcircuit are described, for example, in U.S. Pat. No. 6,140,630; U.S.Pat. No. 6,376,868; U.S. Pat. No. 6,310,366; U.S. Pat. No. 6,326,652;U.S. Pat. No. 6,204,524; and U.S. Pat. No. 6,333,205, all assigned toMicron Technology Inc. The disclosures of each of the foregoing arehereby incorporated by reference herein in their entirety.

A conventional CMOS APS (active pixel sensor) four-transistor (4T) pixelcell 10 is illustrated in FIGS. 1A and 1B. FIG. 1A is a top-down view ofthe cell 10; FIG. 1B is a cross-sectional view of the cell 10 of FIG.1A, taken along line A-A′. The illustrated cell 10 includes a pinnedphotodiode 13 as a photosensor. Alternatively, the CMOS cell 10 mayinclude a photogate, photoconductor or other photon-to-charge convertingdevice, in lieu of the pinned photodiode 13, as the initial accumulatingarea for photo-generated charge. The photodiode 13 includes a p+ surfaceaccumulation region 5 and an underlying n-type accumulation region 14formed in a p-type semiconductor substrate layer 2.

The pixel cell 10 of FIGS. 1A and 1B has a transfer gate 7 fortransferring photocharges generated in the n-type accumulation region 14to a floating diffusion region 3 (i.e., storage region). The floatingdiffusion region 3 is further connected to a gate 27 of a sourcefollower transistor. The source follower transistor provides an outputsignal to a row select access transistor having a gate 37 forselectively gating the output signal to an output terminal (not shown).A reset transistor having a gate 17 resets the floating diffusion region3 to a specified charge level before each charge transfer from then-type accumulation region 14 of the photodiode 13.

The illustrated pinned photodiode 13 is formed in the p-type substrate2. It is also possible, for example, to have a p-type substrate basebeneath p-wells in an n-type epitaxial layer. The n-type accumulationregion 14 and p+ surface accumulation region 5 of the photodiode 13 arespaced between an isolation region 9 and the transfer gate 7. Theillustrated conventional pinned photodiode 13 has a p+/n-/p− structure.

The photodiode 13 has two p-type regions 5, 2 having the same potentialso that the n- accumulation region 14 is fully depleted at a pinningvoltage (V_(pin)). The photodiode 13 is termed “pinned” because thepotential in the photodiode 13 is pinned to a constant value, V_(pin),when the photodiode 13 is fully depleted. When the transfer gate 7 isconductive, photo-generated charge is transferred from the n-accumulating region 14 to the floating diffusion region 3.

Additionally, impurity doped source/drain regions 32, having n-typeconductivity, are provided on either side of the transistor gates 17,27, 37 to produce the reset, source follower, and row selecttransistors, respectively. Conventional processing methods are used toform contacts 33 in an insulating layer to provide an electricalconnection 33 to the source/drain regions 32, the floating diffusionregion 3, and other wiring to connect to the transistor gates 17, 27,and 37 and to form other connections in the cell 10.

Conventional 4T pixel cells, like the one depicted in FIGS. 1A and 1B,have the advantage over 3T pixel cells of having lower fixed patternnoise. The 4T pixel cells, however, have several drawbacks, which arenow discussed generally. First, during the transfer of charges from thephotodiode 13 to the floating diffusion region 3, some charges are leftbehind on the photodiode 13. This incomplete transfer creates lag, andcan also lead to saturation of the photodiode 13 due to the presence ofexcess charge. The traditional 4T design also reduces the fill factor ofthe cell 10 because the four transistors utilize space that couldotherwise be used for a larger photo-sensitive area. As shown in FIG.1A, the conventional pixel cell 10 has approximately a fifty percentfill factor, as only about half of the cell 10 (i.e., photodiode 13)makes up the photo-sensing area.

There is needed, therefore, a pixel cell having low fixed pattern noisebut with a high fill factor, and reduced lag associated with thetransferring of photo-charges. There is also a need for a simple methodof fabricating the desired cell.

BRIEF SUMMARY OF THE INVENTION

Exemplary embodiments of the invention provide an imager pixel cell witha photosensitive JFET structure having a channel region located above aburied charge accumulation region. The channel region has a charge flowcharacteristic that changes depending on the level of accumulated chargein the accumulation region. During an integration period, incident lightcauses electrons to be accumulated inside the buried accumulationregion. The charge flow characteristic of the channel region changes inresponse to a field created by the charges accumulated in theaccumulation region.

In accordance with one aspect of the invention, the pixel cell canperform a charge accumulation mode simultaneously with performing areadout of the pixel, allowing for automatic light control operations.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of the invention will be betterunderstood from the following detailed description of the invention,which is provided in connection with the accompanying drawings, inwhich:

FIG. 1A is a top-down view of a conventional four-transistor (4T) pixelcell;

FIG. 1B is a cross-sectional view of the conventional four-transistorpixel cell of FIG. 1A, taken along line A-A′;

FIG. 2A is a schematic of a circuit diagram of an exemplary pixel cellconstructed in accordance with a first exemplary embodiment of theinvention;

FIG. 2B is a cross-sectional view of the exemplary pixel cell of FIG. 2Aconstructed in accordance with a first exemplary embodiment of theinvention;

FIG. 3 is a flowchart depicting the operation of a pixel cell accordingto an exemplary embodiment of the invention;

FIG. 4 is a block diagram of an imaging device constructed according tothe invention; and

FIG. 5 shows a processor system incorporating at least one imager deviceconstructed in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized, and thatstructural, logical and electrical changes may be made without departingfrom the spirit and scope of the present invention.

The term “substrate” is to be understood as a semiconductor-basedmaterial including silicon, silicon-on-insulator (SOI) orsilicon-on-sapphire (SOS) technology, doped and undoped semiconductors,epitaxial layers of silicon supported by a base semiconductorfoundation, and other semiconductor structures. Furthermore, whenreference is made to a “substrate” in the following description,previous process steps may have been utilized to form regions orjunctions in the base semiconductor structure or foundation. Inaddition, the semiconductor need not be silicon-based, but could bebased on silicon-germanium, germanium, or gallium arsenide.

The term “pixel” refers to a picture element unit cell containing aphotosensor and transistors for converting light radiation to anelectrical signal. For purposes of illustration, a representative pixelis illustrated in the figures and description herein and, typically,fabrication of all pixels in an imager will proceed simultaneously in asimilar fashion.

Although the invention is described herein with reference to thearchitecture and fabrication of one pixel cell, it should be understoodthat this is representative of a plurality of pixel cells in an array ofan imager device such as array 240 of imager device 308 (FIG. 4). Inaddition, the invention has applicability to many solid state imagingdevices having pixel cells, and is not limited to the configurationdescribed herein. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the invention isdefined only by the appended claims.

Referring now to the drawings, where like elements are designated bylike reference numerals, FIG. 2A illustrates in electrical schematicform a pixel cell 100 in accordance with one embodiment of theinvention. As shown, pixel cell 100 includes a light sensitive JFET 107connected in source follower fashion through a row select transistor110. The gate structure of the JFET is light sensitive and serves as acharge collection region for the pixel 100. The gate structure can bereset by a reset transistor 112 which receives a reset control signal(Reset). The row select transistor 110 is responsive to a row selectsignal (RS) at its gate input to thereby couple the pixel 100 to acolumn line 60 for readout of signals from the pixel 100.

FIG. 2B illustrates a cross-sectional view of a pixel cell 100fabricated in accordance with a first exemplary embodiment of theinvention. The pixel cell 100 is formed on a p-type semiconductorsubstrate 101. A photosensitive JFET structure 107 is comprised ofregions 102, 104 formed in the substrate 101 and terminal contacts 105,106. The first region is a doped n-type charge accumulation region 102.The thickness of the depletion region 103 decreases as chargesaccumulate in the charge accumulation region 102, as will be describedin more detail below. A doped p-type channel region 104 is formed in thearea of the substrate 101 above the accumulation region 102 and under atop surface of the substrate 101.

Two terminal contacts 105, 106 are formed on the surface of thesubstrate 101 on opposite sides of channel region 104. The terminalcontacts 105, 106 may be formed of a layer of polysilicon or a suitablemetal contact layer. The first terminal contact 105 is formed over andat one side of the channel region 104. The second terminal contact 106is located at a second side (e.g., at the outermost edge) of the channelregion 104. The first terminal contact is connected to a voltage source,shown as V_(gnd). The bulk of the substrate, which is usually grounded,may be used as a terminal 105 for most implementations. The secondterminal contact 106 is electrically connected to a row selecttransistor 110. The row select transistor 110 is utilized to selectivelyconnect the pixel 100 to readout circuitry during readout of signalsfrom the pixel cell 100.

The illustrated pixel cell 100 has a reset transistor 112 and anassociated reset drain 111. The reset transistor 112 comprises agatestack that may be formed using conventional techniques. For example,the illustrated gatestack has an insulative layer 121 over a conductivelayer 122, which is formed over a gate oxide layer 123 on the surface ofthe substrate 101. The illustrated gatestack also has insulativesidewalls 124, which may be formed of an oxide, nitride, or otherappropriate dielectric material as known in the art. Reset gate 112 whenoperative, couples the charge accumulation region 102, which acts as thegate of JFET 107, to a power supply V_(aa-pix) to drain charges fromregion 102. The exemplary pixel cell 100 also has an STI region 119,which provides isolation from adjacent pixel cells when the pixel cell100 is incorporated into a pixel array 240 (FIG. 5). It should beunderstood that pixel cell 100 may have additional isolation regions andthat other methods of isolation are also within the scope of theinvention. The described reset technique is provided as exemplary, andother embodiments of the invention may utilize different resetapproaches.

The substrate 101 may be doped utilizing an implant of boron ions intothe substrate 101. Although the invention is not in any way limited to aparticular dopant concentration, the concentration of dopant ions in thesubstrate 101 may be in the range of about 1e¹⁵ atoms per cm³ to about1e²⁰ atoms per cm³. The accumulation region 102 is doped n- type byimplanting a suitable dopant ion into a pre-determined area of thesubstrate 101. Suitable ions include antimony, arsenic, and phosphorus.The concentration of dopant ions in the charge accumulation region 102may be in the range of about 1e¹⁵ atoms per cm³ to about 1e²⁰ atoms percm³. The channel region 104 should be lightly doped p- type. Thischannel region 104 is lightly doped with boron ions to a concentrationthat may be in the range of about 1e¹² atoms per cm³ to about 1e¹⁵ atomsper cm³. The drain region 111 should also be lightly doped n-type.

The exemplary pixel cell 100 operates in a charge accumulation modeduring an integration period. During charge accumulation, incident lightis absorbed into the substrate 101. Electron-hole pairs are generated inthe substrate 101 particularly at the junction of oppositely dopedregions (i.e., in the vicinity of the p-n junctions). Electrons arestored in the charge accumulation region 102, while holes are repelledinto the p-type regions such as the channel region 104. These electronsreduce the field existing in the depletion region (due to built-involume charge) that reduces the depletion region 103 thickness andincreases the thickness of the channel region 104. As a result, a chargeflow conductance of the channel region 104 is characteristic of theamount of charges accumulated in the buried accumulation region 102.

A readout of the charges accumulated in the accumulation region 102 isdesired, as the charges correspond to the amount of incident lightapplied to the pixel cell 100. An exemplary readout operation for pixelcell 100 begins by having a pre-determined voltage (e.g., V_(gnd))applied at the first terminal contact 105. A readout of the current istaken from the terminal contact 106 at the other side of the channelregion 104 representing a measurement of the charges collected in theaccumulation region 102. In this way, the JFET structure is operatinglike a source follower transistor 27 (FIG. 1A) for a conventional pixelcell 10. The current flowing through the second terminal contact 106 isselectively read out through the row select transistor 110 whenactivated, and is converted to V_(sig) by readout circuitry (FIG. 4).The readout of pixel cell 100, as just described, is advantageously anon-destructive readout because the charges are not transferred out ofthe accumulation region 102 through a transfer transistor 7 (FIGS. 1Aand 1B) as they are in a conventional 4T pixel cell 10. This allowsmultiple readouts if desired. In addition, this non-destructive readoutallows the pixel cell 100 to be used in an automatic light control (ALC)circuit which continuously reads the incident light. As such, the rowselect transistor 110 remains on to allow a continuous pixel outputsignal V_(sig) to be applied to column line 60 (FIG. 2A). It should alsobe noted that although FIG. 2B shows a voltage applied to a firstcontact 105 and the row select transistor 110 connected to a secondcontact 106, these can be reversed.

Other than the terminal contacts 105, 106 and reset gate 112, the pixelcell 100 has no other structures over the photo-sensitive areas of thesubstrate that could block incoming light, such as a transfer gatestack.Thus, unlike the convention pixel cell 10 (FIG. 1A), which has a fillfactor of approximately fifty percent, the pixel cell 100 has anincreased fill factor.

In order to reset the pixel 100, the reset transistor 112 gate isactivated which turns the reset transistor 112 on to couple the chargeaccumulation region 102 to the voltage supply V_(aa-pix) connected todrain region 111. Thus, a pixel reset output signal, V_(rst) can be readthrough an activated row select transistor 110 after region 102 isreset. When charges accumulate in region 102, the row select transistor110 may be on to supply a continuous pixel output signal V_(sig)sampling at the end of an integration period. Alternatively, the rowselect transistor 110 may be turned on at the end of the integrationperiod to produce V_(sig) output signal for sampling.

Referring to FIGS. 3 and 4, the operation of exemplary pixel cell 100 asincorporated into an imager device 308 (FIG. 4) is now described. Inoperation, the reset transistor 112 resets the charges in theaccumulation region 102 by dumping the charges into the reset drainregion 111, which is connected to a supply voltage. During the resetoperation, the reset transistor 112 is turned on by control circuitry250 (FIG. 4) to allow charges in the accumulation region 102 to bedrained into the drain region 111 (Step 201). Similar to the chargereadout discussed above, a readout of the reset condition (V_(rst))occurs next with sample and hold circuit 261. Specifically, row selecttransistor 110 is turned on, and the charge current traveling throughthe channel region 104 is read out through the second terminal contact106. As approximately no charge is in the accumulation region 102 duringthis reset condition, the readout, at step 202, should reflect aninitial (or unaffected) voltage. An associated sample and hold step 203is performed by sample and hold circuitry 261 (FIG. 4). It should benoted that the accumulation region 102 may be fully depleted during thereset operation just described, thus minimizing any kTC noise in thepixel cell 100.

After charge collection region 102 is reset, an integration periodbegins during which accumulating charges in the accumulation region 102affect the charge current flow in the channel region 104. Since constantcharge monitoring can occur through these terminal contacts 105, 106 asdescribed above as long as row select transistor 110 is on, automaticlight control (ALC) operation may occur as the pixel output signalV_(sig) is continuously output on column line 60. The ALC signal may beread out to determine an optimal time for readout of the entire pixelarray. As shown, in step 204 of FIG. 3, during the ALC operational mode,if implemented, readout of pixel cell 100 output V_(sig) is used as asignal V_(ALC). The signal V_(ALC) can be compared with a predeterminedreference signal V_(trigger) to produce an ALC control signal throughcomparator 251 when the V_(ALC) signal reaches the trigger value. Itshould be understood that the readout of the signal V_(ALC) representsthe amount of charge accumulated in accumulation region 102 at a givenpoint in time.

One use of the V_(ALC) signal can be to stop the integration of an imagewhen a pixel of an array is close to saturation. Thus, when at step 204V_(ALC) reaches the voltage V_(trigger), the image processor 280 (FIG.4) can be informed and can terminate the image integration period.

Whether an ALC operation is employed or not, at the end of anintegration period, the row select transistor 110 is on and the pixeloutput signal V_(sig) is applied to column line 60 (step 205) and issampled and held by sample and hold circuit 261.

Other embodiments of a pixel cell 100 may be constructed in accordancewith the invention. For example, although the exemplary pixels 100, 200have been described as having a p-type substrate 101, n-typeaccumulation region 102, and p-type channel region 104, the invention isnot limited to the described configuration. It should be understood thatother configurations, including a pixel cell having a reversed dopantprofile, are other embodiments that are within the scope of the presentinvention.

FIG. 4 shows an exemplary CMOS imaging integrated circuit 308 whichincludes a pixel array 240, with rows and columns of pixel cells. Asshown in FIG. 4, each pixel of array 240 could be implemented likeexemplary cell 100. The pixels of each row in array 240 are turned by arow select line, illustratively RS (FIG. 2A). All pixels in a row may beturned on at the same time for these operations. Signals from eachcolumn are provided on a respective column line and selectively outputby column selector 260 in response to respective column select signals.The row select lines are selectively activated by a row driver 245 inresponse to a row address decoder 220. The column select lines areselectively activated by a column address decoder 270. Thus, a row andcolumn address is provided for each pixel 100 in the array 240.

The pixel array 240 is operated by the timing and control circuit 250,which controls address decoders 220, 270 for selecting the appropriaterow and column lines for pixel readout and sampling. The control circuit250 also controls the row and column driver circuitry 245, 260 such thatthese apply driving voltages to the drive transistors of the selectedrow and column select lines. Control circuit 250 also controls sampleand hold (S/H) circuit 261 to read and store pixel output signals fromcolumn selector 260. S/H circuit 261 receives pixel reset signal V_(rst)and pixel image signal V_(sig) and provides them to a differentialamplifier 263. A differential signal (V_(sig)-V_(rst)) is produced bydifferential amplifier 263 for each pixel, and the differential signalis then digitized by the analog to digital converter 275 (ADC). Theanalog to digital converter 275 supplies the digitized pixel signals toan image processor 280 which forms and outputs a digital image.

Additional ALC circuitry may also include, in this embodiment, ALCcircuitry 251 for reading the sample voltage, V_(ALC), from a pixel'sphoto-conversion device. V_(ALC) will be sampled either periodically orcontinuously until it approximates the predetermined reference triggervoltage V_(trigger) at which point the ALC monitoring circuit 251produces a signal used by image processor 280. The image processor thensignals timing and control circuitry 250 to initiate a readout process,including readout of V_(rst) and V_(sig) from all of the pixel cells ofarray 240.

A value for V_(trigger) may be selected as desired. For example,V_(trigger) may be chosen such that readout will occur only when pixelcells of array 240 have accumulated sufficient charge to result in animage in which characteristics of the imaged subject matter are visible.Otherwise stated, V_(trigger) may be chosen such that a resultant imagewill not be too dark.

This ALC circuitry 251 just described may be a part of the image sensorintegrated circuit 308 or, alternatively, it may be separate from theimage sensor integrated circuit 308. Without being limiting, forexample, ALC circuitry may be included in the form of hardware orequivalent software in a processor, such as a CPU, which communicateswith the image sensor integrated circuit 308.

FIG. 5 illustrates a processor-based system 1100 including an imagingdevice 308, which has pixels constructed in accordance with the methodsdescribed herein. For example, pixels may be any of the exemplary pixelcells 100 constructed in accordance with the exemplary embodiments ofthe invention described above. The processor-based system 1100 isexemplary of a system having digital circuits that could include imagesensor devices. Without being limiting, such a system could include acomputer system, camera system, scanner, machine vision, vehiclenavigation, video phone, surveillance system, auto focus system, startracker system, motion detection system, image stabilization system, anddata compression system.

The processor-based system 1100, for example a camera system, generallycomprises a central processing unit (CPU) 1102, such as amicroprocessor, that communicates with an input/output (I/O) device 1106over a bus 1104. Imaging device 308 also communicates with the CPU 1102over the bus 1104, and may include a CMOS pixel array having theexemplary pixels 100 as discussed above. The processor-based system 1100also includes random access memory (RAM) 1110, and can include removablememory 1115, such as flash memory, which also communicates with CPU 1102over the bus 1104. Imaging device 308 may be combined with a processor,such as a CPU, digital signal processor, or microprocessor, with orwithout memory storage on a single integrated circuit or on a differentchip than the processor. Any of the memory storage devices in theprocessor-based system 1100 could store software for employing theabove-described method.

The above description and drawings are only to be consideredillustrative of exemplary embodiments which achieve the features andadvantages of the invention. Modification of, and substitutions to,specific process conditions and structures can be made without departingfrom the spirit and scope of the invention. Accordingly, the inventionis not to be considered as being limited by the foregoing descriptionand drawings, but is only limited by the scope of the appended claims.

1-31. (canceled)
 32. A method of forming a pixel sensor comprising stepsof: forming a photosensitive JFET having a gate region buried beneath atop surface of a substrate; forming a channel region located above thegate region; and forming circuitry electrically coupled to the channelregion for reading out a signal representing the amount of chargesgenerated by the photosensitive JFET.
 33. The method of claim 32,wherein the step of forming circuitry comprises forming a first terminalat the surface of the substrate adjacent a first side of the channelregion.
 34. The method of claim 33, wherein the step of formingcircuitry further comprises the step of: forming a second terminal atthe surface of the substrate and located adjacent a second side of thechannel region.
 35. The method of claim 34, wherein the step of forminga first terminal comprises forming a layer of polysilicon.
 36. Themethod of claim 32, wherein the channel region is a p-type regionlocated in the substrate and the gate region is an n-type chargeaccumulation region that accumulates the amount of charges generated bythe photosensitive JFET. 37-47. (canceled)
 48. A method of forming aphotosensor, comprising: forming a light striking region; forming a JFETchannel region; and forming a JFET gate region comprising anaccumulation region for changing a resistance of the JFET channel regionby accumulating charges generated in response to light received by thelight striking region.
 49. The method of claim 48, wherein theaccumulation region is doped to a first conductivity type and the JFETchannel region is doped to a second conductivity type.
 50. The method ofclaim 48, wherein an increase in the charges accumulated by theaccumulation region causes an increase in conductivity of the JFETchannel region.
 51. The method of claim 50, wherein the JFET channelregion and the JFET gate region are buried beneath a substrate surfaceof the photosensor.
 52. A method of forming a pixel sensor cell,comprising: forming a photosensitive element for generating charges inresponse to applied light; forming an accumulation region foraccumulating an amount of charges generated by the photosensitiveelement; and forming a JFET channel region having a changeableresistance dependent upon the amount of charges in the accumulationregion.
 53. The method of claim 52, further comprising: forming acircuit for producing a signal based on the resistance of the channelregion.
 54. The method of claim 53, wherein the JFET channel region isformed beneath a top surface of the substrate and above the chargeaccumulation region.
 55. The method of claim 52, wherein an increase inthe amount of charges accumulated by the accumulation region causes anincrease in conductivity of the JFET channel region.
 56. The method ofclaim 52, wherein the amount charges generated by the photosensitiveelement can be determined by the resistance of the JFET channel region.57. The method of claim 52, further comprising: forming a first contactfor applying a voltage to the JFET channel region; and forming a secondcontact for outputting a signal corresponding to the resistance of theJFET channel region when the first contact applies the voltage.
 58. Themethod of claim 57, further comprising: forming a row select transistorelectrically connected to the second contact.
 59. The method of claim57, further comprising: forming a drain for releasing the amount ofcharges accumulated by the accumulation region to start a reset period.60. The method of claim 59, further comprising: forming sample and holdcircuitry for receiving a signal based on the resistance of the JFETchannel region during the reset period.
 61. A method of forming a pixelsensor cell, comprising: forming a photosensitive element at leastpartially in a substrate, the photosensitive element for generating anamount of charges in response to applied light; forming a JFET channelregion located beneath the top surface of the substrate; forming a JFETgate region beneath a top surface of the substrate and under the JFETchannel region, the JFET gate region including a charge accumulationregion adapted to vary a resistance characteristic of the JFET channelregion by accumulating the amount of charges generated by thephotosensitive element; forming a first contact electrically connectedto the JFET channel region and for reading out a signal representing acurrent flowing through the channel region; and forming a transistor fordraining the accumulated charges from the charge accumulation region toa drain region.
 62. A method of forming an array of pixel sensor cells,comprising: forming a plurality of pixel sensor cells in a substrate;forming an accumulation region in at least one pixel cell, theaccumulation region for accumulating an amount of charges generated by aphotosensor of the at least one pixel cell and located beneath a topsurface of the substrate; and forming a JFET channel region in the atleast one pixel cell, the JFET channel region located above theaccumulation region and having a resistance adapted to change inresponse to the amount of charges accumulated in the accumulationregion.
 63. The array of pixel sensor cells of claim 62, wherein the atleast one pixel cell further comprises a first terminal contact applyinga voltage to the JFET channel region and a second terminal contactperforming a read out in response to the applied voltage.
 64. The arrayof pixel sensor cells of claim 63, wherein the at least one pixel cellfurther comprises a first row select transistor selectively reading outsignals from the second terminal contact.